Wavelength division multiplexing/demultiplexing devices using dual high index of refraction crystalline lenses

ABSTRACT

A wavelength division multiplexing device is disclosed. In one embodiment, the wavelength division multiplexing device comprises a crystalline collimating lens for collimating a plurality of monochromatic optical beams, a diffraction grating for combining the plurality of collimated, monochromatic optical beams into a multiplexed, polychromatic optical beam, and a crystalline focusing lens for focusing the multiplexed, polychromatic optical beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part patent application ofU.S. patent application Ser. No. 08/990,199, filed Dec. 13, 1997, nowU.S. Pat. No. 5,999,672 which is hereby incorporated by reference hereinin its entirety.

This patent application is also a continuation-in-part patentapplication of U.S. patent application Ser. No. 09/323,094, filed Jun.1, 1999, which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates generally to wavelength divisionmultiplexing/demultiplexing and, more particularly, to wavelengthdivision multiplexing/demultiplexing devices using dual high index ofrefraction crystalline lenses.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) is a rapidly emerging technologythat enables a very significant increase in the aggregate volume of datathat can be transmitted over optical fibers. Prior to the use of WDM,most optical fibers were used to unidirectionally carry only a singledata channel at one wavelength. The basic concept of WDM is to launchand retrieve multiple data channels in and out, respectively, of anoptical fiber. Each data channel is transmitted at a unique wavelength,and the wavelengths are appropriately selected such that the channels donot interfere with each other, and the optical transmission losses ofthe fiber are low. Today, commercial WDM systems exist that allow forthe transmission of 2 to 100 simultaneous data channels.

WDM is a cost-effective method of increasing the volume of data(commonly termed bandwidth) transferred over optical fibers. Alternatecompeting technologies for increasing bandwidth include the burying ofadditional fiber optic cable or increasing the optical transmission rateover optical fiber. The burying of additional fiber optic cable is quitecostly as it is presently on the order of $15,000 to $40,000 perkilometer. Increasing the optical transmission rate is limited by thespeed and economy of the electronics surrounding the fiber optic system.One of the primary strategies for electronically increasing bandwidthhas been to use time division multiplexing (TDM), which groups ormultiplexes multiple lower rate electronic data channels together into asingle very high rate channel. This technology has for the past 20 yearsbeen very effective for increasing bandwidth. However, it is nowincreasingly difficult to improve transmission speeds, both from atechnological and an economical standpoint. WDM offers the potential ofboth an economical and technological solution to increasing bandwidth byusing many parallel channels.

Further, WDM is complimentary to TDM. That is, WDM can allow manysimultaneous high transmission rate TDM channels to be passed over asingle optical fiber.

The use of WDM to increase bandwidth requires two basic devices that areconceptually symmetrical. The first device is a wavelength divisionmultiplexer. This device takes multiple beams, each with discretewavelengths that are initially spatially separated in space, andprovides a means for spatially combining all of the different wavelengthbeams into a single polychromatic beam suitable for launching into anoptical fiber. The multiplexer may be a completely passive opticaldevice or may include electronics that control or monitor theperformance of the multiplexer. The input to the multiplexer istypically accomplished with optical fibers, although laser diodes orother optical sources may also be employed. As mentioned above, theoutput from the multiplexer is a single polychromatic beam which istypically directed into an optical fiber.

The second device for WDM is a wavelength division demultiplexer. Thisdevice is functionally the opposite of the wavelength divisionmultiplexer. That is, the wavelength division demultiplexer receives apolychromatic beam from an optical fiber and provides a means ofspatially separating the different wavelengths of the polychromaticbeam. The output from the demultiplexer is a plurality of monochromaticbeams which are typically directed into a corresponding plurality ofoptical fibers or photodetectors.

During the past 20 years, various types of WDMs have been proposed anddemonstrated. For example, (1) W. J. Tomlinson, Applied Optics, Vol. 16,No. 8, pp. 2180-2194 (August 1977); (2) A. C. Livanos et al., AppliedPhysics Letters, Vol. 30, No. 10, pp. 519-521 (May 15, 1977); (3) H.Ishio et al., Journal of Lightwave Technology, Vol 2, No. 4, pp. 448-463(August 1984); (4) H. Obara et al., Electronics Letters, Vol. 28, No.13, pp. 1268-1270 (Jun. 18, 1992); (5) A. E. Willner et al., IEEEPhotonics Technology Letters, Vol. 5, No. 7, pp. 838-841 (July 1993);and Y. T. Huang et al., Optical Letters, Vol. 17, No. 22, pp. 1629-1631(Nov. 15, 1992), all disclose some form of WDM device and/or method.However, all of the above-listed approaches employ lenses formed ofstandard optical glass materials such as, for example, BK7, FK3, andSF6. While these standard optical glass materials are adequate for usewith multimode optical fibers, they are generally inadequate for usewith single mode optical fibers because the core diameter of a singlemode optical fiber (i.e., typically 8 μm) is much smaller than the corediameter of a multimode optical fiber (i.e., typically 62.5 μm). Thatis, WDM devices employing lenses formed of standard optical glassmaterials have heretofore be unable to receive and transmit opticalbeams from and to single mode optical fibers, respectively, withoutincurring unacceptable amounts of insertion loss and channel crosstalk.These unacceptable levels of insertion loss and channel crosstalk arelargely due to the inadequate imaging capabilities of the lenses formedof standard optical glass materials.

One proposed solution to the above-described optical imaging problem hasbeen to add additional lenses formed of standard optical glass materialsto the WDM devices, thereby resulting in WDM devices having doublet,triplet, and even higher number lens configurations. By adding theseadditional lenses to the WDM devices, wherein the added lenses typicallyhave alternating high and low refraction indexes, aberrations causedmainly by the spherical nature of the lenses are effectively canceledout. However, an increased cost is associated with adding theseadditional lenses due to the direct cost of the additional lenses, aswell as the indirect costs associated with the increased complexity andresulting decreased manufacturability of WDM devices having multiplelenses.

Another proposed solution to the above-described optical imaging problemhas been to use gradient refractive index lenses (e.g., Gradium lenses)in the WDM devices. The use of these gradient refractive index lensesresults in a significant improvement in the quality of the imagingsystem within the WDM devices. However, costs associated withmanufacturing these gradient refractive index lenses is significantlygreater than the costs associated with manufacturing standardhomogeneous lenses, despite the fact that both are typically formed ofstandard optical glass materials.

In view of the foregoing, there remains a real need for a WDM devicewhich possesses or allows for all the characteristics of: low cost,component integration, environmental and thermal stability, low channelcrosstalk, low channel signal loss, ease of interfacing, large number ofchannels, and narrow channel spacing. Accordingly, it would be desirableto provide a WDM device which overcomes the above-described inadequaciesand shortcomings, while possessing or allowing for all of theabove-stated characteristics.

OBJECTS OF THE INVENTION

The primary object of the present invention is to provide wavelengthdivision multiplexing/demultiplexing devices which use dual high indexof refraction crystalline lenses to achieve increased deviceperformance, as well as reduced device cost, complexity, andmanufacturing risk.

The above-stated primary object, as well as other objects, features, andadvantages, of the present invention will become readily apparent fromthe following detailed description which is to be read in conjunctionwith the appended drawings.

SUMMARY OF THE INVENTION

According to the present invention, a wavelength division multiplexingdevice is provided. In one embodiment, the wavelength divisionmultiplexing device comprises a crystalline collimating lens forcollimating a plurality of monochromatic optical beams, a diffractiongrating for combining the plurality of collimated, monochromatic opticalbeams into a multiplexed, polychromatic optical beam, and a crystallinefocusing lens for focusing the multiplexed, polychromatic optical beam.

The crystalline collimating lens and the crystalline focusing lens arepreferably plano-convex lenses, or bi-convex lenses, although other lensconfigurations are possible. For example, the crystalline collimatinglens and the crystalline focusing lens can be spherical or aspherical.Also, the crystalline collimating lens and the crystalline focusing lenshave high indexes of refraction and preferably operate in the infrared(IR) region of the electromagnetic spectrum since this is the regionwhere the power loss (attenuation) and dispersion of silica-basedoptical fibers is very low. Accordingly, the crystalline collimatinglens and the crystalline focusing lens are typically formed ofcrystalline materials selected from the group consisting of silicon,germanium, gallium arsenide, zinc sulfide, cadmium sulfide, zincselenide, and cadmium selenide, as well as from any of a number of otherappropriate high index of refraction crystalline materials thatefficiently transmit optical beams in the infrared (IR) region of theelectromagnetic spectrum. The diffraction grating is preferably atransmissive diffraction grating.

In accordance with other aspects of the present invention, thewavelength division multiplexing device is provided in integrated form.That is, an integrated wavelength division multiplexing device isprovided comprising a crystalline collimating lens for collimating aplurality of monochromatic optical beams, a first homogeneous index bootlens affixed to the crystalline collimating lens for transmitting theplurality of collimated, monochromatic optical beams from thecrystalline collimating lens, wherein the first homogeneous index bootlens has a planar exit surface, and a diffraction grating formed at theplanar exit surface of the first homogeneous index boot lens forcombining the plurality of collimated, monochromatic optical beams intoa multiplexed, polychromatic optical beam.

In accordance with further aspects of the present invention, theintegrated wavelength division multiplexing device may include a secondhomogeneous index boot lens affixed to the crystalline collimating lensfor transmitting the plurality of monochromatic optical beams to thecrystalline collimating lens. The second homogeneous index boot lenspreferably has a planar entry surface for accepting the plurality ofmonochromatic optical beams from at least one optical source. (eg.,optical fibers, laser diodes) Alternatively, the integrated wavelengthdivision multiplexing device may do without the second homogeneous indexboot lens, and the crystalline collimating lens can have a planar entrysurface for accepting the plurality of monochromatic optical beams fromat least one optical source. (e.g. optical fibers, photodetectors)

In accordance with still further aspects of the present invention, thediffraction grating can be a transmissive diffraction grating and theintegrated wavelength division multiplexing device may include atransmissive element associated with the transmissive diffractiongrating, wherein the transmissive element preferably has at least onereflective surface for reflecting the multiplexed, polychromatic opticalbeam. The integrated wavelength division multiplexing device may thenalso include a second homogeneous index boot lens affixed to thetransmissive element for transmitting the multiplexed, polychromaticoptical beam from the transmissive element, and a crystalline focusinglens affixed to the second homogeneous index boot lens for focusing themultiplexed, polychromatic optical beam. The integrated wavelengthdivision multiplexing device may then further include a thirdhomogeneous index boot lens affixed to the crystalline focusing lens fortransmitting the focused, multiplexed, polychromatic optical beam fromthe crystalline focusing lens. The third homogeneous index boot lenspreferably has a planar exit surface for outputting the focused,multiplexed, polychromatic optical beam to at least one optical receiver(e.g., optical fibers, photodetectors). Alternatively, the integratedwavelength division multiplexing device may do without the thirdhomogeneous index boot lens, and the crystalline focusing lens can havea planar exit surface for outputting the focused, multiplexed,polychromatic optical beam to at least one optical receiver. (e.g.,optical fibers photodectors)

In accordance with other aspects of the present invention, thewavelength division multiplexing device is provided in an alternativeintegrated form. That is, an alternative integrated wavelength divisionmultiplexing device is provided comprising a crystalline focusing lensfor focusing a multiplexed, polychromatic optical beam, a firsthomogeneous index boot lens affixed to the crystalline focusing lens fortransmitting the multiplexed, polychromatic optical beam to thecrystalline focusing lens, wherein the first homogeneous index boot lenshas a planar entry surface, and a diffraction grating formed at theplanar entry surface of the first homogeneous index boot lens forcombining a plurality of monochromatic optical beams into themultiplexed, polychromatic optical beam.

In accordance with further aspects of the present invention, thediffraction grating can be a transmissive diffraction grating and thealternative integrated wavelength division multiplexing device mayinclude a transmissive element associated with the transmissivediffraction grating, wherein the transmissive element preferably has atleast one reflective surface for reflecting the plurality ofmonochromatic optical beams toward the transmissive diffraction grating.The alternative integrated wavelength division multiplexing device maythen also include a second homogeneous index boot lens affixed to thetransmissive element for transmitting the plurality of monochromaticoptical beams to the transmissive element, and a crystalline collimatinglens affixed to the second homogeneous index boot lens for collimatingthe plurality of monochromatic optical beams prior to transmission bythe second homogeneous index boot lens.

At this point it should be noted that the above-described wavelengthdivision multiplexing device, integrated wavelength divisionmultiplexing device, and alternative integrated wavelength divisionmultiplexing device are all bidirectional devices. Thus, the wavelengthdivision multiplexing device can also be a wavelength divisiondemultiplexing device, the integrated wavelength division multiplexingdevice can also be an integrated wavelength division demultiplexingdevice, and the alternative integrated wavelength division multiplexingdevice can also be an alternative integrated wavelength divisiondemultiplexing device. Further, all of the above-described devices canbe used simultaneously as both a multiplexer and a demultiplexer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIG. 1a is a side view of a wavelength division multiplexing devicehaving dual plano-convex crystalline collimating/focusing lenses and atransmissive diffraction grating in accordance with the presentinvention.

FIG. 1b is a perspective end view of a portion of the wavelengthdivision multiplexing device shown in FIG. 1a.

FIG. 2a is a perspective view of a coupling device containing aplurality of laser diodes for replacing the plurality of optical inputfibers in the multiplexing device shown in FIG. 1a.

FIG. 2b is a perspective view of a coupling device containing aplurality of photodetectors for replacing the plurality of optical inputfibers in the demultiplexing device shown in FIG. 3.

FIG. 3 is a side view of a wavelength division demultiplexing devicehaving dual plano-convex crystalline collimating/focusing lenses and atransmissive diffraction grating in accordance with the presentinvention.

FIG. 4 is a side view of an integrated wavelength division multiplexingdevice having dual plano-convex crystalline collimating/focusing lensesand a transmissive diffraction grating in accordance with the presentinvention.

FIG. 5 is a side view of an integrated wavelength division multiplexingdevice having dual extended plano-convex crystallinecollimating/focusing lenses and a transmissive diffraction grating inaccordance with the present invention.

FIG. 6 is a side view of a wavelength division multiplexing devicehaving dual bi-convex crystalline collimating/focusing lenses and atransmissive diffraction grating in accordance with the presentinvention.

FIG. 7 is a side view of an integrated wavelength division multiplexingdevice having dual bi-convex crystalline collimating/focusing lenses anda transmissive diffraction grating in accordance with the presentinvention.

FIG. 8 is a side view of an integrated in-line wavelength divisionmultiplexing device having dual bi-convex crystalline lenses, atransmissive diffraction grating, and a reflecting element in accordancewith the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1a, there is shown a side view of a preferredembodiment of a wavelength division multiplexing device 10 in accordancewith the present invention. The multiplexing device 10 comprises aplurality of optical input fibers 12, an input fiber coupling device 14,a plano-convex crystalline collimating lens 16, a reflecting element 18having a reflecting surface 18 a, a transmissive diffraction grating 20,a plano-convex crystalline focusing lens 22, an output fiber couplingdevice 24, and a single optical output fiber 26.

At this point it should be noted that the optical input fibers 12 andthe optical output fiber 26, as well as any other optical fibersdescribed herein as being used in conjunction with WDM devices inaccordance with the present invention, are single mode optical fibers.Of course, however, this does not limit the present invention WDMdevices to use with only single mode optical fibers. For example, thepresent invention WDM devices can also be used with multimode opticalfibers.

It should also be noted that the multiplexing device 10, as well as anyother WDM devices described herein as being WDM devices in accordancewith the present invention, is operating in the infrared (IR) region ofthe electromagnetic spectrum as a dense wavelength division multiplexing(DWDM) device (i.e., operating with data channels having channelspacings of 1 nm or less). Of course, however, this does not limit thepresent invention WDM devices to being only DWDM devices. For example,the present invention WDM devices can also be standard WDM devices(i.e., operating with data channels having channel spacings greater than1 nm).

Returning to FIG. 1a, the plurality of optical input fibers 12 aregrouped into a one-dimensional input fiber array (i.e., a 1×4 array) bythe input fiber coupling device 14, while the single optical outputfiber 26 is secured to the output fiber coupling device 24. Both theinput fiber coupling device 14 and the output fiber coupling device 24are used for purposes of ease of optical fiber handling and precisionplacement, and can be formed of, for example, a silicon V-grooveassembly. Referring to FIG. 1b, there is shown a perspective end view ofa portion of the multiplexing device 10 revealing how the plurality ofoptical input fibers 12 are grouped into the one-dimensional input fiberarray by the input fiber coupling device 14, and how the single opticaloutput fiber 26 is secured to the output fiber coupling device 24. FIG.1b also shows a monochromatic optical input beam 28 being transmittedfrom each of the plurality of optical input fibers 12, and a singlemultiplexed, polychromatic optical output beam 30 being transmitted tothe single optical output fiber 26.

Each of the monochromatic optical input beams 28 being transmitted fromthe plurality of optical input fibers 12 is carrying a single channel ofdata at a unique wavelength, which is preferably, but not required tobe, within the infrared (IR) region of the electromagnetic spectrum. Thesingle channel of data that is being carried by each monochromaticoptical input beam 28 is superimposed on each corresponding uniquewavelength by means (e.g., laser diodes connected to the plurality ofoptical input fibers 12), which are not shown here and which do not forma part of this invention, but are well known in the art. The uniquewavelengths of the monochromatic optical input beams 28 areappropriately preselected such that the data channels do not interferewith each other (i.e., there is sufficient channel spacing), and theoptical transmission losses through both the optical input fibers 12 andthe optical output fiber 26 are low, as is also well known in the art.

The single multiplexed, polychromatic optical output beam 30 beingtransmitted to the single optical output fiber 26 is carrying aplurality of channels of data at the unique wavelengths of each of theplurality of monochromatic optical input beams 28. The plurality ofmonochromatic optical input beams 28 are combined into the singlemultiplexed, polychromatic optical output beam 30 through the combinedoperation of the plano-convex crystalline collimating lens 16, thetransmissive diffraction grating 20, and the plano-convex crystallinefocusing lens 22, as will be described in more detail below.

Referring again to FIG. 1a, each of the plurality of monochromaticoptical input beams 28 are transmitted from their corresponding opticalinput fiber 12 into the air space between the input fiber couplingdevice 14 and the planoconvex crystalline collimating lens 16. Withinthis air space, the plurality of monochromatic optical input beams 28are expanded in diameter until they become incident upon theplano-convex crystalline collimating lens 16. The planoconvexcrystalline collimating lens 16 collimates each of the plurality ofmonochromatic optical input beams 28, and then transmits each of aplurality of collimated, monochromatic optical input beams 28′ to thereflecting element 18.

The reflecting element 18 is fabricated of a transmissive material suchas, for example, optical glass. Thus, each of the plurality ofcollimated, monochromatic optical input beams 28′ is transmitted throughthe reflecting element 18 toward the reflecting surface 18 a, which isformed at a reflecting angle, θ, on a beveled edge of the reflectingelement 18. The reflecting surface 18 a reflects each of the pluralityof collimated, monochromatic optical input beams 28′ such that aplurality of reflected, collimated, monochromatic optical input beams28″ are transmitted through the reflecting element 18 toward thetransmissive diffraction grating 20. The reflecting angle, θ, is chosenbased upon the desired center wavelength diffraction angle of thetransmissive diffraction grating 20, as will be described in more detailbelow.

The transmissive diffraction grating 20 operates to angularly dispersethe plurality of reflected, collimated, monochromatic optical inputbeams 28″ by an amount that is dependent upon the wavelength of each ofthe plurality of reflected, collimated, monochromatic optical inputbeams 28″. That is, the transmissive diffraction grating 20 operatesaccording to the well known diffraction grating equation,

mλ=d(sin α+sin β)

wherein m is the diffraction order, λ is the wavelength, d is thediffraction grating groove spacing, α is the incident angle with respectto the diffraction grating normal, and β is the diffraction angle withrespect to the diffraction grating normal. For the multiplexing device10 shown in FIG. 1a, the diffraction angle, β, is desired to be 0°, sothe incident angle, α, is equal to 45° for a center wavelength of 1550nm and a diffraction grating having an order of 1 and a groove spacingof 0.65 μm. The reflecting angle, θ, is equal to one-half of theincident angle, α, for the multiplexing device 10 shown in FIG. 1a. Sothe reflecting angle, θ, is equal to 22.50 for the multiplexing device10 shown in FIG. 1a. Of course, the present invention is not limited tothe values just described as they are provided for purposes ofillustration only.

At this point it should be noted that the transmissive diffractiongrating 20 can be formed from a variety of materials and by a variety oftechniques. For example, the transmissive diffraction grating 20 can beformed by a three-dimensional hologram in a polymer medium, or by amechanical ruling on a planar material such as, for example, glass orsilicon. The transmissive diffraction grating 20 could then be joined oraffixed to the surface of the reflecting element 18 using optical cementor some other optically transparent bonding technique. Alternatively,the transmissive diffraction grating 20 can be formed directly on thesurface of the reflecting element 18, thereby avoiding the joining oraffixing of the transmissive diffraction grating 20 to the surface ofthe reflecting element 18.

As previously mentioned, the transmissive diffraction grating 20operates to angularly disperse the plurality of reflected, collimated,monochromatic optical input beams 28″. Thus, the transmissivediffraction grating 20 removes the angular separation of the pluralityof reflected, collimated, monochromatic optical input beams 28″, andtransmits a single collimated, polychromatic optical output beam 30′towards the plano-convex crystalline focusing lens 22. The singlecollimated, polychromatic optical output beam 30′ contains each of theunique wavelengths of the plurality of reflected, collimated,monochromatic optical input beams 28″. Thus, the single collimated,polychromatic optical output beam 30′ is a single collimated,multiplexed, polychromatic optical output beam 30′. The plano-convexcrystalline focusing lens 22 focuses the single collimated, multiplexed,polychromatic optical output beam 30′, and then transmits the resultingsingle multiplexed, polychromatic optical output beam 30 to the outputfiber coupling device 24 where it becomes incident upon the singleoptical output fiber 26. The single multiplexed, polychromatic opticaloutput beam 30 is then coupled into the single optical output fiber 26for transmission therethrough.

At this point it should be noted that the single multiplexed,polychromatic optical output beam 30 is insured of being directed to thesingle optical output fiber 22 in a very efficient manner (i.e., withvery low insertion losses and negligible channel crosstalk) by virtue ofthe enhanced imaging of both the input optical beams 28 and outputoptical beam 30 within the multiplexing device 10 through the use of theplano-convex crystalline collimating lens 16 and the plano-convexcrystalline focusing lens 22. This enhanced imaging of both the inputoptical beams 28 and output optical beam 30 within the multiplexingdevice 10 is a direct result of the plano-convex crystalline collimatinglens 16 and the plano-convex crystalline focusing lens 22 being formedof a high index of refraction crystalline material.

The use of a high index of refraction crystalline material to form theplano-convex crystalline collimating lens 16 and the plano-convexcrystalline focusing lens 22 insures that the multiplexing device 10operates in a very efficient manner (i.e., with very low insertionlosses and negligible channel crosstalk) due to the fact that a largedifference exists between the high index of refraction of theplanoconvex crystalline collimating lens 16 and the plano-convexcrystalline focusing lens 22 and the much lower index of refraction ofthe air spaces adjacent to these lenses 16, 22. This large differencebetween the high index of refraction of the plano-convex crystallinecollimating lens 16 and the plano-convex crystalline focusing lens 22and the much lower index of refraction of the adjacent air spaces allowsfor the highly efficient collimation and focusing of the input opticalbeams 28 and output optical beam 30 by the plano-convex crystallinecollimating lens 16 and the plano-convex crystalline focusing lens 22,respectively, while simultaneously minimizing the amount of wavelengthdistortion that is introduced into the optical system of themultiplexing device 10 by these lenses 16, 22. Furthermore, this largedifference between the high index of refraction of the planoconvexcrystalline collimating lens 16 and the plano-convex crystallinefocusing lens 22 and the much lower index of refraction of the adjacentair spaces is much greater than can be achieved using lenses formed ofstandard optical glasses because standard optical glasses have index ofrefraction values that are much lower than high index of refractioncrystalline materials. Thus, the efficiencies that are achieved by usinga high index of refraction crystalline material to form the plano-convexcrystalline collimating lens 16 and the plano-convex crystallinefocusing lens 22 are greater than can be achieved using lenses formed ofstandard optical glasses.

Examples of high index of refraction crystalline materials which can beused to form the plano-convex crystalline collimating lens 16 and theplano-convex crystalline focusing lens 22 include silicon (Si), galliumarsenide (GaAs), germanium (Ge), zinc sulfide (ZnS), cadmium sulfide(CDs), zinc selenide (ZnSe), cadmium selenide (CdSe), and any of anumber of other appropriate high index of refraction crystallinematerials that efficiently transmit optical beams in the infrared (IR)region of the electromagnetic spectrum, since this is the region wherethe power loss (attenuation) and dispersion of silica-based opticalfibers is very low. In fact, most WDM devices are used in the window of1530-1610 nm, which is the range over which erbium-doped fiberamplifiers (EDFAs) operate and optical fibers have low loss. This1530-1610 nm region is often called the “third window” for opticalfibers. Similarly, however, some WDM devices are also used in theso-called “second window” for optical fibers (i.e., typically within thewindow of 1300-1330 nm) where optical fibers have very low dispersionand low loss. Consequently, most prior art WDM devices use standardoptical glasses that transmit efficiently in these IR regions. Forexample, standard optical glasses such as BK7, FK3, SF6, and Gradiumhave optical transmission efficiencies of 97-99% for one-inch materialthicknesses in these IR regions. This level of transmission efficiencyis generally adequate, but, as previously mentioned, there are costconsiderations associated with the use of these materials for lenses inWDM devices (i.e., increased component costs for WDM devices requiringmultiple lenses formed of standard optical glass materials, andincreased fabrication costs for gradient refractive index lenses).Furthermore, all of these standard optical glasses have index ofrefraction values (i.e., typically n<2.0) that are much lower than highindex of refraction crystalline materials (e.g., for silicon, n=3.46@1550 nm).

An additional benefit to using a high index of refraction crystallinematerial to form the plano-convex crystalline collimating lens 16 andthe plano-convex crystalline focusing lens 22 is that the use of acrystalline lens allows the collimating lens 16 and the focusing lens 22to be a plano-convex singlet instead of a bi-convex singlet, doublet, oreven higher number lens configuration. This means that thecollimating/focusing power of only one curved surface on a crystallinelens is sufficient to provide essentially diffraction-limitedcollimating/focusing. The relative amount of curvature needed in atypical crystalline lens is very low, and as a result the resultingspherical aberration is very low, almost negligible. It should be noted,however, that the above does not preclude the collimating lens 16 andthe focusing lens 22 from being a bi-convex crystallinecollimating/focusing singlet, doublet, or even higher number lensconfiguration. To the contrary, if the collimating lens 16 or thefocusing lens 22 is a bi-convex crystalline collimating/focusingsinglet, doublet, or even higher number lens configuration, the imagingof both the input optical beams 28 and output optical beam 30 within themultiplexing device 10 is improved even more, as will be discussed inmore detail below.

A further benefit to using a high index of refraction crystallinematerial to form the plano-convex crystalline collimating lens 16 andthe plano-convex crystalline focusing lens 22 is that the high index ofrefraction crystalline material can be used to lessen, and possibly eveneliminate, aberrations caused by the spherical nature of the lenses 16,22. These aberrations are lessened because the much greater refractiveindex of the high index crystalline material allows the radius of theplano-convex crystalline collimating lens 16 and the plano-convexcrystalline focusing lens 22 to be greatly increased (i.e., the lenseshave much less curvature), thereby resulting in much less spherical andother aberrations. For example, if the plano-convex crystallinecollimating lens 16 or the plano-convex crystalline focusing lens 22were to be fabricated of silicon (e.g., n=3.46), then, everything elseremaining the same, the required radius of the lenses 16, 22 would bemuch greater (i.e., the lenses would have less curvature or be lesssteep) than if the lenses 16, 22 were to be fabricated of a typicaloptical glass (e.g., LaSF 18A manufactured by Schott Glass Technologies,Inc. with n=1.872) due to the large difference between the refractiveindex values of silicon and air (i.e., 3.46−1.0=2.46) in comparison tothe lesser difference between the refractive index values of LaSF 18Aand air (i.e., 1.872−1.0=0.872). That is, the difference between therefractive index values of silicon and air is 2.82 times greater thanthe difference between the refractive index values of LaSF 18A and air.Accordingly, the radius of the lenses 16, 22 if fabricated of silicon isallowed to be 2.82 times greater than the radius of the lenses 16, 22 iffabricated of LaSF 18A. Further, aberrations caused by the sphericalnature of the lenses 16, 22 are also typically reduced by this samefactor (i.e., by 2.82 times).

The above-described ability to decrease the level of aberrations in themultiplexing device 10 by using a high index of refraction crystallinematerial to form the plano-convex crystalline collimating lens 16 andthe plano-convex crystalline focusing lens 22 is very significant. Thisdiscovery insures that the use of high index of refraction materialswill result in a very large amount (or degree) of lens design freedom.The high index of refraction can be used either to make the curvature ofa lens less steep, or to simplify the number and/or complexity of thelenses that are used in a WDM device.

At this point it should be noted that the plano-convex crystallinecollimating lens 16 and the plano-convex crystalline focusing lens 22,as well as any other crystalline collimating/focusing lens describedherein as being used in WDM devices in accordance with the presentinvention, may be spherical or aspherical in shape. Although sphericallenses are more common than aspherical lenses, mainly due to the factthat they are easier to manufacture, the performance of a WDM device maybe further improved by using an aspherical crystallinecollimating/focusing lens instead of a spherical crystallinecollimating/focusing lens. That is, the curvature at the edges of anaspherical crystalline collimating/focusing lens is less steep than thecurvature at the edges of a spherical crystalline collimating/focusinglens, thereby resulting in even further reductions in the level ofspherical aberrations in a WDM device incorporating such an asphericalcrystalline collimating/focusing lens.

At this point it should also be noted that the planoconvex crystallinecollimating lens 16 and the plano-convex crystalline focusing lens 22,as well as any other crystalline collimating/focusing lens describedherein as being used in WDM devices in accordance with the presentinvention, is typically coated with an anti-reflection material due tothe high index of refraction of the crystalline material.

At this point it should be noted that the plurality of optical inputfibers 12 could be replaced in the multiplexing device 10 by acorresponding plurality of laser diodes 32 secured within a couplingdevice 34, such as shown in FIG. 2a. The coupling device 34 performs asimilar function to the input fiber coupling device 14, that being toprecisely group the plurality of laser diodes 32 into a one-dimensionalinput array. The plurality of laser diodes 32 are used in place of theplurality of optical input fibers 12 to transmit the plurality ofmonochromatic optical input beams 28 to the multiplexing device 10. Thearray of laser diodes 32 may operate alone, or may be used withappropriate focusing lenses to provide the best coupling and the lowestamount of signal loss and channel crosstalk.

At this point it should be noted that the multiplexing device 10, aswell as all of the multiplexing devices described herein, may beoperated in a converse configuration as a demultiplexing device 40, suchas shown in FIG. 3. The demultiplexing device 40 is physically identicalto the multiplexing device 10, and is therefore numerically identifiedas such. However, the demultiplexing device 40 is functionally oppositeto the multiplexing device 10, wherein the plano-convex crystallinecollimating lens 16 now functions as a plano-convex crystalline focusinglens 16 and the planoconvex crystalline focusing lens 22 now functionsas a planoconvex crystalline collimating lens 22. That is, a singlemultiplexed, polychromatic optical input beam 42 is transmitted from thesingle optical fiber 22, and a plurality of monochromatic optical outputbeams 44 are transmitted to the plurality of optical fibers 12, whereineach one of the plurality of monochromatic optical output beams 44 istransmitted to a corresponding one of the plurality of optical fibers12. The single multiplexed, polychromatic optical input beam 42 issimultaneously carrying a plurality of channels of data, each at aunique wavelength which is preferably, but not required to be, withinthe infrared (IR) region of the electromagnetic spectrum. The pluralityof monochromatic optical output beams 44 are each carrying a singlechannel of data at a corresponding one of the unique wavelengths of thesingle multiplexed, polychromatic optical input beam 42. In this case,the single multiplexed, polychromatic optical input beam 42 is separatedinto the plurality of monochromatic optical output beams 44 through thecombined operation of the plano-convex crystalline collimating lens 22,the transmissive diffraction grating 20, and the plano-convexcrystalline focusing lens 16. That is, the plano-convex crystallinecollimating lens 22 collimates the single multiplexed, polychromaticoptical input beam 42 to provide a single collimated, multiplexed,polychromatic optical input beam 42′. The transmissive diffractiongrating 20 spatially separates the single collimated, multiplexed,polychromatic optical input beam 42′ into a plurality of collimated,monochromatic optical input beams 44″, which are reflected off thereflecting surface 18 a to provide a plurality of reflected, collimated,monochromatic optical input beams 44′. The plano-convex crystallinefocusing lens 16 focuses the plurality of reflected, collimated,monochromatic optical input beams 44′ to provide the plurality ofmonochromatic optical output beams 44. Thus, the plano-convexcrystalline collimating lens 22, the transmissive diffraction grating20, and a plano-convex crystalline focusing lens 16 operate to perform ademultiplexing function. Of course, in this case, the incident angle, α,and the diffraction angle, β, are reversed in comparison to themultiplexing device 10 shown in Figure la, and the reflecting angle, θ,is equal to one-half of the diffraction angle, β.

At this point it should be noted that the plurality of optical fibers 12could be replaced in the demultiplexing device 40 by a correspondingplurality of photodetectors 36 secured within a coupling device 38, suchas shown in FIG. 2b. The coupling device 38 performs a similar functionto the fiber coupling device 14, that being to precisely group theplurality of photodetectors 36 into a one-dimensional output array. Theplurality of photodetectors 36 are used in place of the plurality ofoptical fibers 12 to receive the plurality of monochromatic opticaloutput beams 44 from the demultiplexing device 40. The array ofphotodetectors 36 may operate alone, or may be used with appropriatefocusing lenses to provide the best coupling and the lowest amount ofsignal loss and channel crosstalk.

Referring to FIG. 4, there is shown a side view of an alternateembodiment of a wavelength division multiplexing device 50 in accordancewith the present invention. The multiplexing device 50 is physicallyidentical to the multiplexing device 10, except for the addition of afirst homogeneous index boot lens 52 between the input fiber couplingdevice 14 and the plano-convex crystalline collimating lens 16, a secondhomogeneous index boot lens 54 and an optional spacer 21 between theplano-convex crystalline collimating lens 16 and the reflecting element18, a third homogeneous index boot lens 56 between the transmissivediffraction grating 20 and the plano-convex crystalline focusing lens22, and a fourth homogeneous index boot lens 58 between the plano-convexcrystalline focusing lens 22 and the output fiber coupling device 24.The first homogeneous index boot lens 52, the second homogeneous indexboot lens 54, the third homogeneous index boot lens 56, and the fourthhomogeneous index boot lens 58 are preferably fabricated, for example,of fused silica (n=1.444), although numerous other optical glassmaterials may also be used. The optional spacer 21 is preferablyfabricated of an optical glass such as, for example, fused silica, andis used to maintain the spacing and alignment between the various partsof the multiplexing device 50.

The first homogeneous index boot lens 52 has a planar front surface 52 afor mating with the input fiber coupling device 14 and the associatedsecured optical input fibers 12. The input fiber coupling device 14 andthe secured optical input fibers 12 may be either abutted against theplanar front surface 52 a or affixed to the planar front surface 52 ausing optical cement or some other optically transparent bondingtechnique, depending upon system mobility requirements and optical beamalignment and loss considerations.

The first homogeneous index boot lens 52 also has a planar back surface52 b for mating with the planar surface of the plano-convex crystallinecollimating lens 16, while the second homogeneous index boot lens 54 hasa concave front surface 54 a for mating with the convex surface of theplanoconvex crystalline collimating lens 16. The plano-convexcrystalline collimating lens 16 is typically joined or affixed to boththe planar back surface 52 b of the first homogeneous index boot lens 52and the concave front surface 54 a of the second homogeneous index bootlens 54 using optical cement or some other optically transparent bondingtechnique.

The second homogeneous index boot lens 54 also has a planar back surface54 b for mating with a planar front surface 21 a of the optional spacer21, while the optional spacer 21 also has a planar back surface 21 b formating with a planar interface surface 18 b of the reflecting element18. The optional spacer 21 is typically joined or affixed to both theplanar back surface 54 b of the second homogeneous index boot lens 54and the planar interface surface 18 b of the reflecting element 18 usingoptical cement or some other optically transparent bonding technique.

Similar to the first homogeneous index boot lens 52, the fourthhomogeneous index boot lens 58 has a planar front surface 58 a formating with the output fiber coupling device 24 and the associatedsecured optical output fiber 26. The output fiber coupling device 24 andthe secured optical output fiber 26 may be either abutted against theplanar front surface 58 a or affixed to the planar front surface 58 ausing optical cement or some other optically transparent bondingtechnique, depending upon system mobility requirements and optical beamalignment and loss considerations.

The fourth homogeneous index boot lens 58 also has a planar back surface58 b for mating with the planar surface of the plano-convex crystallinefocusing lens 22, while the third homogeneous index boot lens 56 has aconcave front surface 56 a for mating with the convex surface of theplano-convex crystalline focusing lens 22. The plano-convex crystallinefocusing lens 22 is typically joined or affixed to both the planar backsurface 58 b of the fourth homogeneous index boot lens 58 and theconcave front surface 56 a of the third homogeneous index boot lens 56using optical cement or some other optically transparent bondingtechnique.

The third homogeneous index boot lens 56 also has a planar back surface56 b for typically mating with a planar front surface 20 a of thetransmissive diffraction grating 20, while the transmissive diffractiongrating 20 typically has a planar back surface 20 b for mating with theplanar interface surface 18 b of the reflecting element 18. Thetransmissive diffraction grating 20 is then typically joined or affixedto both the planar back surface 56 b of the third homogeneous index bootlens 56 and the planar interface surface 18 b of the reflecting element18 using optical cement or some other optically transparent bondingtechnique. However, as with the multiplexing device 10, the transmissivediffraction grating 20 can be formed directly on the planar interfacesurface 18 b of the reflecting element 18, or on the planar back surface56 b of the third homogeneous index boot lens 56, thereby avoiding thejoining or affixing of the transmissive diffraction grating 20 to thereflecting element 18 or the third homogeneous index boot lens 56,respectively, and also avoiding the need for the optional spacer 21.

In either of the above-described cases, the transmissive diffractiongrating 20 is integrated along with the reflecting element 18, theplano-convex crystalline collimating lens 16, the plano-convexcrystalline focusing lens 22, and the homogeneous index boot lenses 52,54, 56, and 58, to form a compact, rigid, and environmentally andthermally stable multiplexing device 50. The integrated nature of thismultiplexing device 50 is particularly useful for maintaining componentalignment, which provides long-term performance in contrast to somenon-integrated air-spaced devices that characteristically degrade inalignment and therefore performance over time.

The multiplexing device 50 is functionally identical to the multiplexingdevice 10, except for a slight decrease in optical beam transmissionefficiency due to the addition of the homogeneous index boot lenses 52,54, 56, and 58. However, even with this slight decrease in optical beamtransmission efficiency, the optical performance of the multiplexingdevice 50 is still exceptional due to the use of a high index ofrefraction crystalline material to form the plano-convex crystallinecollimating lens 16 and the plano-convex crystalline focusing lens 22.That is, as previously described, the high index of refractioncrystalline material can be used to lessen, and possibly even eliminate,aberrations caused by the spherical nature of the lenses 16, 22. Andthese aberrations are still lessened despite the addition of thehomogeneous index boot lenses 52, 54, 56, and 58. For example, if thehomogeneous index boot lenses 52, 54, 56, and 58 were to be fabricatedof a first type of standard optical glass (e.g., FK3 manufactured bySchott Glass Technologies, Inc. with n=1.450) and if the plano-convexcrystalline collimating lens 16 and the plano-convex crystallinefocusing lens 22 were to be fabricated of silicon (e.g., n=3.46), then,everything else remaining the same, the required radius of the lenses16, 22 would be much greater (i.e., the lenses would have less curvatureor be less steep) than if the lenses 16, 22 were to be fabricated of asecond type of standard optical glass (e.g., LaSF 18A manufactured bySchott Glass Technologies, Inc. with n=1.872) due to the largedifference between the refractive index values of silicon and FK3 (i.e.,3.46−1.450=2.01) in comparison to the lesser difference between therefractive index values of LaSF 18A and FK3 (i.e., 1.872−1.450=0.422).That is, the difference between the refractive index values of siliconand FK3 is 4.76 times greater than the difference between the refractiveindex values of LaSF 18A and FK3. Accordingly, the radius of the lenses16, 22 if fabricated of silicon is allowed to be 4.76 times greater thanthe radius of the lenses 16, 22 if fabricated of LaSF 18A. Further,aberrations caused by the spherical nature of the lenses 16, 22 are alsotypically reduced by this same factor (i.e., by 4.76 times).

Referring to FIG. 5, there is shown a side view of an alternateembodiment of a wavelength division multiplexing device 60 in accordancewith the present invention. The multiplexing device 60 is physicallyidentical to the multiplexing device 50, except that the firsthomogeneous index boot lens 52 has been removed and the planar frontsurface 16′a of the plano-convex crystalline collimating lens 16′ hasbeen extended so as to allow the input fiber coupling device 14 and thesecured optical input fibers 12 to be either abutted against the planarfront surface 16′a or affixed to the planar front surface 16′a usingoptical cement or some other optically transparent bonding technique,depending upon system mobility requirements and optical beam alignmentand loss considerations, and that the fourth homogeneous index boot lens58 has been removed and the planar front surface 22′a of theplano-convex crystalline focusing lens 22′ has been extended so as toallow the output fiber coupling device 24 and the secured optical outputfiber 26 to be either abutted against the planar front surface 22′a oraffixed to the planar front surface 22′a using optical cement or someother optically transparent bonding technique, depending upon systemmobility requirements and optical beam alignment and lossconsiderations. The multiplexing device 60 is functionally identical tothe multiplexing device 50, except for a slight increase in optical beamtransmission efficiency due to the removal of the first homogeneousindex boot lens 52 and the fourth homogeneous index boot lens 58.

At this point it should be noted that any of the homogeneous index bootlenses 52, 54, 56, and 58 may be removed from the multiplexing device50, and either of the homogeneous index boot lenses 54, 56 may beremoved from the multiplexing device 60 in order to create additionalalternate embodiments (not shown) while still retaining theabove-described benefits of using a high index of refraction crystallinematerial to form the plano-convex crystalline collimating lens 16 andthe plano-convex crystalline focusing lens 22.

Referring to FIG. 6, there is shown a side view of an alternateembodiment of a wavelength division multiplexing device 70 in accordancewith the present invention. The multiplexing device 70 is physicallyidentical to the multiplexing device 10, except that the plano-convexcrystalline collimating lens 16 has been replaced by a bi-convexcrystalline collimating lens 72 and that the planoconvex crystallinefocusing lens 22 has been replaced by a biconvex crystalline focusinglens 74 so as to further enhance the imaging of both the input opticalbeams 28 and output optical beam 30 within the multiplexing device 70.That is, the additional curved surfaces of the bi-convex crystallinecollimating lens 72 and the bi-convex crystalline focusing lens 74provides additional imaging capability, thereby increasing the fibercoupling efficiency (FCE) of the multiplexing device 70. In contrast toa measure of insertion loss, the FCE of a WDM device expresses theefficiency of only the optical system of the WDM device for each datachannel, without taking into account the efficiency of the diffractiongrating. Comparatively, the use of the bi-convex crystalline collimatinglens 72 and the bi-convex crystalline focusing lens 74 instead of theplano-convex crystalline collimating lens 16 and the plano-convexcrystalline focusing lens 22, respectively, typically results in anincrease in the FCE of approximately 1% for the configuration of WDMdevices shown in FIGS. 1 and 6. Thus, a trade-off must be made between asmall increase in the FCE and the additional cost associated withfabricating lenses having an additional curved surface. Of course,further increases in the FCE can typically be achieved using doublet,triplet, or even higher number lens configurations.

Referring to FIG. 7, there is shown a side view of an alternateembodiment of a wavelength division multiplexing device 80 in accordancewith the present invention. The multiplexing device 80 is physicallyidentical to the multiplexing device 50, except that the plano-convexcrystalline collimating lens 16 has been replaced by a biconvexcrystalline collimating lens 72, the first homogeneous index boot lens52 has been replaced by a first homogeneous index boot lens 82, theplano-convex crystalline focusing lens 22 has been replaced by abi-convex crystalline focusing lens 74, and the fourth homogeneous indexboot lens 58 has been replaced by a fourth homogeneous index boot lens84. As with the multiplexing device 70, the replacement of theplanoconvex crystalline collimating lens 16 with the bi-convexcrystalline collimating lens 72 and the replacement of the plano-convexcrystalline focusing lens 22 with the bi-convex crystalline focusinglens 74 in the multiplexing device 80 has been done to further enhancethe imaging of both the input optical beams 28 and output optical beam30 within the multiplexing device 80. The first homogeneous index bootlens 52 has been replaced with the first homogeneous index boot lens 82because the first homogeneous index boot lens 82 has a concave backsurface 82 b for mating with the convex front surface 72 a of thebi-convex crystalline collimating lens 72. Similarly, the fourthhomogeneous index boot lens 58 has been replaced with the fourthhomogeneous index boot lens 84 because the fourth homogeneous index bootlens 84 has a concave back surface 84 b for mating with the convex frontsurface 74 a of the bi-convex crystalline focusing lens 74.

At this point it should be noted that, as with the multiplexing device50, any of the homogeneous index boot lenses 82, 54, 56, and 84 may beremoved from the multiplexing device 80 in order to create additionalalternate embodiments (not shown) while still retaining theabove-described benefits of using a high index of refraction crystallinematerial to form the bi-convex crystalline collimating lens 72 and thebiconvex crystalline focusing lens 74. Also, the bi-convex crystallinecollimating lens 72 and/or the bi-convex crystalline focusing lens 74can be replaced with a planoconvex crystalline lens(es), or acrystalline collimating/focusing doublet, triplet, or even higher numberlens configuration, in the multiplexing device 80 in accordance with thepractices described above.

Referring to FIG. 8, there is shown a side view of an alternateembodiment of a wavelength division multiplexing device 90 in accordancewith the present invention. The multiplexing device 90 differs from thepreviously described embodiments by having an in-line geometry ratherthan the folded geometry of the previously described embodiments. Thisin-line geometry is achieved through the use of a dual reflectingelement 92, which has a first reflecting surface 92 a for reflecting theplurality of collimated, monochromatic optical input beams 28′ and asecond reflecting surface 92 b for reflecting the plurality ofreflected, collimated, monochromatic optical input beams 28″. Note that,in contrast to the previously described embodiments, the multiplexingdevice 90 does not require the optional spacer 21 to maintain thespacing and alignment between the various parts of the multiplexingdevice 90. Otherwise, the multiplexing device 90 is functionallyidentical to and utilizes all of the components used in the multiplexingdevice 80, except of course the reflecting element 18.

At this point it should be noted that, as with the multiplexing device50 and the multiplexing device 80, any of the homogeneous index bootlenses 82, 54, 56, and 84 may be removed from the multiplexing device 90in order to create additional alternate embodiments (not shown) whilestill retaining the above-described benefits of using a high index ofrefraction crystalline material to form the bi-convex crystallinecollimating lens 72 and the bi-convex crystalline focusing lens 74.Also, the bi-convex crystalline collimating lens 72 and/or the bi-convexcrystalline focusing lens 74 can be replaced with a plano-convexcrystalline lens(es), or a crystalline collimating/focusing doublet,triplet, or even higher number lens configuration, in the multiplexingdevice 90 in accordance with the practices described above. The benefitsand detriments associated with using these substitute/additionalcomponents are applicable to the multiplexing device 90 as would be thecase with the previously described embodiments. Of course, the mostsignificant benefits come from the use of high index of refractioncrystalline materials for the lenses. That is, regardless of embodiment,the use of high index of refraction crystalline materials for lenses inWDM devices yields increased device performance, as well as reduceddevice cost, complexity, and manufacturing risk. Simply said, the use ofhigh index of refraction crystalline lenses allows for the constructionof a family of simple, low cost, yet very powerful WDM devices,particularly for use in DWDM (i.e., high channel number) applications.

In addition to those discussed above, there are other advantages tousing high index of refraction crystalline materials rather thanstandard optical glass materials for lenses in WDM devices. Below is adiscussion of some of these advantages.

1. High Refractive Index: As mentioned above, materials such as silicon(Si), germanium (Ge), gallium arsenide (GaAs), zinc sulfide (ZnS),cadmiun sulfide (CdS), zinc selenide (ZnSe), cadmiun selenide (CdSe), orany of a number of other appropriate high index of refractioncrystalline materials all have refractive indices that are higher thanany practically available optical glass materials. The use of high indexof refraction crystalline materials gives more “horsepower” to a lensdesign. That is, the use of high index of refraction crystallinematerials insures that there is a large amount of additional designflexibility available to either lower the aberrations in a WDM device,simplify the complexity of the lenses used in a WDM device, or extendthe capabilities of a WDM device (e.g., by adding additional datachannels). Compared to the use of optical glass lenses, crystallinelenses allow a WDM device to have equivalent or better opticalperformance, as typically measured by the fiber coupling efficiency(FCE) of the WDM device.

2. Refractive Index Accuracy: Crystalline lenses by nature of beingcrystalline are ultra accurate and precise in their refractive index anddispersion values. For example, the refractive index of a crystallinematerial is typically constant from batch to batch to better than 1 partin the 4^(th) decimal place (e.g., <3.XXXX±0.0001). This is the inherentnature of crystalline materials. This refractive index precisionessentially eliminates design and manufacturing problems caused byvariations in materials. However, depending upon the processing method,certain crystalline materials can be alloyed or contain impurities, butthis does not appear to be a significant concern for the use of siliconwhich is commonly and economically prepared at a very high level ofpurity for the semiconductor industry. Compared to optical glass lenses,crystalline lenses have about 10 to 100 times more precise refractiveindex and dispersion values. For example, it is common to use an Hlgrade of optical glass, which has a refractive index accuracy of+/−0.001 (melt-to-melt variation) and a precision (part-to-partvariation) of +/−0.00002 within any portion of a single lens or lensblank. Similarly, gradient refractive index glass (e.g., Gradium glass)generally has a refractive index accuracy of not less than 0.0006(melt-to-melt variation).

3. Commercial Availability: Crystalline materials such as silicon (Si),germanium (Ge), gallium arsenide (GaAs), zinc sulfide (ZnS), cadmiunsulfide (CdS), zinc selenide (ZnSe), and cadmiun selenide (CdSe) areavailable from multiple sources. Thus, the use of crystalline materialstypically does not give rise to single-source problems. On the otherhand, optical glasses are presently commercially available from 3 majorcompanies: Schott Glass Technologies, Ohara Corporation, and CorningFrance. Typically, however, glass of the same type from one company isdifferent when obtained from another company. Therefore, optical glassis typically a single source material. This problem can be, and oftenis, minimized to a degree by long production runs and by maintaining aninventory of material. Gradient refractive index glass (e.g., Gradiumglass) is presently produced by a single source vendor (LightPathTechnologies, Inc.), and is not presently inventoried in significantvolumes.

4. Procurement Lead Times: Some crystalline materials require fairlylong lead times to procure. However, silicon is often available frominventory or with a relatively short lead-time. Optical glasses areavailable from inventory about half of the time, but otherwise must beimported or ordered from the next melting cycle. Gradient refractiveindex glass (e.g., Gradium glass) are typically manufactured to order.

5. Lens Fabrication Issues: The fabrication of crystalline IR materialsinto lenses is fairly common, but probably less so than traditionaloptical glasses. Presently, approximately half of all lens fabricatorswork with silicon and/or other crystalline IR materials. Silicon is themost common crystalline IR material fabricated into lenses. There are noknow serious issues related to the grinding and polishing of thismaterial. The grinding and polishing techniques and slurries used forcrystalline materials are different than those for optical glasses.Further, crystalline IR materials have a crystallographic orientationthat must be considered when lenses are fabricated, but this is commonlyunderstood and typically not an issue. In comparison, optical glassmaterials are typically isotropic. Approximately 10 of the 40 largestlens fabrication houses in the U.S. are presently capable of fabricatingGradium glass lenses. The fabrication of Gradium glass lenses iscomplicated due to the necessity of both keeping the gradient axisproperly oriented and the necessity of thinning the glass down to withinapproximately 50 microns of the proper position in the refractive indexprofile. The proper thinning of the Gradium glass requires specialattention by the lens fabricator and increases the fabrication cost of alens by 30-100% over the cost of fabricating a comparable homogeneouslens.

6. Material and Lens Fabrication Costs: The costs of crystalline IRmaterials can be slightly higher than optical glasses, with the notableexception being silicon. Since silicon is widely used in thesemiconductor industry and for other IR optics applications, the pricefor silicon is fairly low and constant, sometimes even lower thanoptical glass materials. For example, silicon lenses nominally 2 cm indiameter cost (material and fabrication costs combined) approximately$200 per lens and about $50 per 100 lenses. This is very nearly the samecost for most homogeneous optical glass lenses. Gradium glass lenses areapproximately $400 per lens in small quantities and not less than $200in large volumes.

7. Ease of Lens Design: The same procedures and lens design codes can beused for the design of crystalline IR materials as for homogenousoptical glasses. Lens designs that use Gradium glasses requireray-tracing code that can properly perform gradient refracting index raytracing. Typically, the added computation complexity of gradient indexray tracing is 3 to 10 times greater than for designs using crystallineor homogeneous glass lenses.

8. Lens Radius Tolerances: In general, the tolerance for a singleplano-convex crystalline lens is typically tighter than for multiplehomogenous lenses. For example, a plano-convex silicon singlet-baseddesign typically requires a tighter tolerance of 1 fringe×½ waveirregularity. This is due to the single curved surface on the siliconlens being responsible for all of the collimation/focusing. However, thetighter tolerance on the plano-convex silicon lens is not significantlymore costly or difficult to fabricate. Further, multiple optical glasslenses are typically required to achieve the performance obtained with asingle crystalline lens.

9. Thermal Expansion: The thermal expansion of silicon is only2.3×10⁻⁶/° C. This is about 3.5 times lower than typical homogeneousoptical glasses and Gradium glasses. The lower expansion of siliconcould be very desirable for a very low expansion WDM device. By reducingthe thermal expansion, the WDM device has less dimensional change withtemperature, and this results in less insertion loss or crosstalksensitivity to temperature change, which can be very desirable. Also,the construction of a low expansion WDM device can allow the device tobe used over a much wider temperature range potentially without the needfor a thermoelectric cooler or other heater type of device. Typically,in homogeneous glass and Gradium glass based approaches, a WDM devicecomprises other spacer glasses which have similar expansion to theglasses used in the collimating lens assembly (typically 8-9×10⁻⁶). Theuse of silicon allows low expansion glasses and glass-ceramic materialssuch as Pyrex, Zerodur, Fused Silica, Fused Quartz, Pyroceram,Clearceram, etc. to be used for the glass spacers. The use of the lowexpansion materials also helps eliminate any thermal stresses related tothe use of a silicon v-groove, which also has low expansion.

10. Optical Assembly: Perhaps the most important point here is thatthere are no unusual problems with either cementing homogenous glass tosilicon or cementing an antireflection coated silicon lens to ahomogenous lens. In fact, several glasses having coefficients of thermalexpansion 2 to 3 times higher than silicon have been cemented to siliconwithout any unusual problems.

11. Thermal Conductivity: The thermal conductivity of silicon is veryhigh, with a value of 200 W/m-K. Common optical glasses have thermalconductivities within the range of 0.7 to 1.1 W/m-K.

12. Ease of Rapid Prototyping and Product Development: The highuniformity of crystalline materials and the ease of performing the lensdesign allows for rapid prototyping of a WDM containing a crystallinelens. Although this is also true for the use of homogeneous lenses, itis not for the use of Gradium glasses. For example, for a WDM devicesuch as disclosed herein, new gradient refractive index profiles wouldbe necessary to obtain the correct level of aberration correction.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, various modifications of thepresent invention, in addition to those described herein, will beapparent to those of skill in the art from the foregoing description andaccompanying drawings. Thus, such modifications are intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A wavelength division multiplexing devicecomprising: a crystalline collimating lens for collimating a pluralityof monochromatic optical beams; a diffraction grating for combining theplurality of collimated, monochromatic optical beams into a multiplexed,polychromatic optical beam; and a crystalline focusing lens for focusingthe multiplexed, polychromatic optical beam.
 2. The device of claim 1,wherein the diffraction grating is a transmissive diffraction grating.3. The device of claim 1, wherein the crystalline collimating lens andthe crystalline focusing lens have high indexes of refraction.
 4. Thedevice of claim 1, wherein the crystalline collimating lens and thecrystalline focusing lens operate in the infrared region of theelectromagnetic spectrum.
 5. The device of claim 1, wherein at least oneof the crystalline collimating lens and the crystalline focusing lens isa plano-convex lens.
 6. The device of claim 1, wherein at least one ofthe crystalline collimating lens and the crystalline focusing lens is abi-convex lens.
 7. The device of claim 1, wherein at least one of thecrystalline collimating lens and the crystalline focusing lens is aspherical lens.
 8. The device of claim 1, wherein at least one of thecrystalline collimating lens and the crystalline focusing lens is anaspherical lens.
 9. The device of claim 1, wherein the crystallinecollimating lens and the crystalline focusing lens are formed of acrystalline material selected from the group consisting of silicon,germanium, gallium arsenide, zinc sulfide, cadmium sulfide, zincselenide, and cadmium selenide.
 10. An integrated wavelength divisionmultiplexing device comprising: a crystalline collimating lens forcollimating a plurality of monochromatic optical beams; a homogeneousindex boot lens affixed to the crystalline collimating lens fortransmitting the plurality of collimated, monochromatic optical beamsfrom the crystalline collimating lens, the homogeneous index boot lenshaving a planar exit surface; and a diffraction grating formed at theplanar exit surface of the homogeneous index boot lens for combining theplurality of collimated, monochromatic optical beams into a multiplexed,polychromatic optical beam.
 11. The device of claim 10, wherein thehomogeneous index boot lens is a first homogeneous index boot lens, thedevice further comprising: a second homogeneous index boot lens affixedto the crystalline collimating lens for transmitting the plurality ofmonochromatic optical beams to the crystalline collimating lens.
 12. Thedevice of claim 11, wherein the second homogeneous index boot lens has aplanar entry surface for accepting the plurality of monochromaticoptical beams from at least one optical source.
 13. The device of claim10, wherein the crystalline collimating lens has a planar entry surfacefor accepting the plurality of monochromatic optical beams from at leastone optical source.
 14. The device of claim 10, wherein the diffractiongrating is a transmissive diffraction grating.
 15. The device of claim14, further comprising: a transmissive element associated with thetransmissive diffraction grating, the transmissive element having atleast one reflective surface for reflecting the multiplexed,polychromatic optical beam.
 16. The device of claim 15, wherein thehomogeneous index boot lens is a first homogeneous index boot lens, thedevice further comprising: a second homogeneous index boot lens affixedto the transmissive element for transmitting the multiplexed,polychromatic optical beam from the transmissive element; and acrystalline focusing lens affixed to the second homogeneous index bootlens for focusing the multiplexed, polychromatic optical beam.
 17. Thedevice of claim 16, further comprising: a third homogeneous index bootlens affixed to the crystalline focusing lens for transmitting thefocused, multiplexed, polychromatic optical beam from the crystallinefocusing lens.
 18. The device of claim 17, wherein the third homogeneousindex boot lens has a planar exit surface for outputting the focused,multiplexed, polychromatic optical beam to at least one opticalreceiver.
 19. The device of claim 16, wherein the crystalline focusinglens has a planar exit surface for outputting the focused, multiplexed,polychromatic optical beam to at least one optical receiver.
 20. Anintegrated wavelength division multiplexing device comprising: acrystalline focusing lens for focusing a multiplexed, polychromaticoptical beam; a homogeneous index boot lens affixed to the crystallinefocusing lens for transmitting the multiplexed, polychromatic opticalbeam to the crystalline focusing lens, the homogeneous index boot lenshaving a planar entry surface; and a diffraction grating formed at theplanar entry surface of the homogeneous index boot lens for combining aplurality of monochromatic optical beams into the multiplexed,polychromatic optical beam.
 21. The device of claim 20, wherein thediffraction grating is a transmissive diffraction grating.
 22. Thedevice of claim 21, further comprising: a transmissive elementassociated with the transmissive diffraction grating, the transmissiveelement having at least one reflective surface for reflecting theplurality of monochromatic optical beams toward the transmissivediffraction grating.
 23. The device of claim 22, wherein the homogeneousindex boot lens is a first homogeneous index boot lens, the devicefurther comprising: a second homogeneous index boot lens affixed to thetransmissive element for transmitting the plurality of monochromaticoptical beams to the transmissive element; and a crystalline collimatinglens affixed to the second homogeneous index boot lens for collimatingthe plurality of monochromatic optical beams prior to transmission bythe second homogeneous index boot lens.
 24. A wavelength divisiondemultiplexing device comprising: a crystalline collimating lens forcollimating a multiplexed, polychromatic optical beam; a diffractiongrating for separating the collimated, multiplexed, polychromaticoptical beam into a plurality of monochromatic optical beams; and acrystalline focusing lens for focusing the plurality of monochromaticoptical beams.
 25. The device of claim 24, wherein the diffractiongrating is a transmissive diffraction grating.
 26. The device of claim24, wherein the crystalline collimating lens and the crystallinefocusing lens have high indexes of refraction.
 27. The device of claim24, wherein the crystalline collimating lens and the crystallinefocusing lens operate in the infrared region of the electromagneticspectrum.
 28. The device of claim 24, wherein at least one of thecrystalline collimating lens and the crystalline focusing lens is aplano-convex lens.
 29. The device of claim 24, wherein at least one ofthe crystalline collimating lens and the crystalline focusing lens is abi-convex lens.
 30. The device of claim 24, wherein at least one of thecrystalline collimating lens and the crystalline focusing lens is aspherical lens.
 31. The device of claim 24, wherein at least one of thecrystalline collimating lens and the crystalline focusing lens is anaspherical lens.
 32. The device of claim 24, wherein the crystallinecollimating lens and the crystalline focusing lens are formed of acrystalline material selected from the group consisting of silicon,germanium, gallium arsenide, zinc sulfide, cadmium sulfide, zincselenide, and cadmium selenide.
 33. An integrated wavelength divisiondemultiplexing device comprising: a crystalline collimating lens forcollimating a multiplexed, polychromatic optical beam; a homogeneousindex boot lens affixed to the crystalline collimating lens fortransmitting the collimated, multiplexed, polychromatic optical beamfrom the crystalline collimating lens, the homogeneous index boot lenshaving a planar exit surface; and a diffraction grating formed at theplanar exit surface of the homogeneous index boot lens for separatingthe collimated, multiplexed, polychromatic optical beam into a pluralityof monochromatic optical beams.
 34. The device of claim 33, wherein thehomogeneous index boot lens is a first homogenous index boot lens, thedevice further comprising: a second homogeneous index boot lens affixedto the crystalline collimating lens for transmitting the multiplexed,polychromatic optical beam to the crystalline collimating lens.
 35. Thedevice of claim 34, wherein the second homogeneous index boot lens has aplanar entry surface for accepting the multiplexed, polychromaticoptical beam from at least one optical source.
 36. The device of claim33, wherein the crystalline collimating lens has a planar entry surfacefor accepting the multiplexed, polychromatic optical beam from at leastone optical source.
 37. The device of claim 33, wherein the diffractiongrating is a transmissive diffraction grating.
 38. The device of claim37, further comprising: a transmissive element associated with thetransmissive diffraction grating, the transmissive element having atleast one reflective surface for reflecting the plurality ofmonochromatic optical beams.
 39. The device of claim 38, wherein thehomogeneous index boot lens is a first homogeneous index boot lens, thedevice further comprising: a second homogeneous index boot lens affixedto the transmissive element for transmitting the plurality ofmonochromatic optical beams from the transmissive element; and acrystalline focusing lens affixed to the second homogeneous index bootlens for focusing the plurality of monochromatic optical beams.
 40. Thedevice of claim 39, further comprising: a third homogeneous index bootlens affixed to the crystalline focusing lens for transmitting theplurality of focused, monochromatic optical beams from the crystallinefocusing lens.
 41. The device of claim 40, wherein the third homogeneousindex boot lens has a planar exit surface for outputting the pluralityof focused, monochromatic optical beams to at least one opticalreceiver.
 42. The device of claim 39, wherein the crystalline focusinglens has a planar exit surface for outputting the plurality of focused,monochromatic optical beams to at least one optical receiver.
 43. Anintegrated wavelength division demultiplexing device comprising: acrystalline focusing lens for focusing a plurality of monochromaticoptical beams; a homogeneous index boot lens affixed to the crystallinefocusing lens for transmitting the plurality of monochromatic opticalbeams to the crystalline focusing lens, the homogeneous index boot lenshaving a planar entry surface; and a diffraction grating formed at theplanar entry surface of the homogeneous index boot lens for separating amultiplexed, polychromatic optical beam into the plurality ofmonochromatic optical beams.
 44. The device of claim 43, wherein thediffraction grating is a transmissive diffraction grating.
 45. Thedevice of claim 44, further comprising: a transmissive elementassociated with the transmissive diffraction grating, the transmissiveelement having at least one reflective surface for reflecting themultiplexed, polychromatic optical beam toward the transmissivediffraction grating.
 46. The device of claim 45, wherein the homogeneousindex boot lens is a first homogenous index boot lens, the devicefurther comprising: a second homogeneous index boot lens affixed to thetransmissive element for transmitting the multiplexed, polychromaticoptical beam to the transmissive element; and a crystalline collimatinglens affixed to the second homogeneous index boot lens for collimatingthe multiplexed, polychromatic optical beam prior to transmission by thesecond homogeneous index boot lens.